Setting Up a Biological Filter

Guest Author - D. J. Herda

If you have a pond, you should be using biological filters to help keep the water fresh. If you have fish, you absolutely should be using them. Here’s why.

Fish, like most living things, produce toxins in the form of ammonia (NH3). They excrete it through their gills and in their waste. If too much ammonia builds up in your pond, your fish’s gills will become irritated. Long-term exposure will eventually damage a fish’s internal organs and result in death.

In nature, the aerobic bacteria, Nitrosonomas, remove the hydrogen and add oxygen to each ammonia molecule, thus converting it chemically from NH3, or ammonia, into NO2, or nitrite. Another bacteria, Nitrobacter, then converts nitrite into nitrate (NO3), which is much less toxic to fish.

The process of converting ammonia into nitrites and nitrites into nitrates is called the "Nitrogen Cycle."

Biological filtration works by attracting and holding both Nitrosonomas and Nitrobacter bacteria. When exposed to oxygenated water, these two bacteria begin converting ammonia into nitrites and nitrites into nitrates. To be effective, a biofilter must have a large surface area on which the bacteria can form. This surface area is known as biomedia, literally a medium on which beneficial bacteria live.

In the past, many materials have been used as biomedia. These include plastic tubing, orange bags, lava rock, foam sheets, and even hair curlers. The problem is that most of these materials take up a great deal of space while providing very little surface area for the bacteria, meaning that your filter box is going to have to be immense in size to provide enough growing surface for the bacteria you need to do the job.

In our ponds, we use a combination of flexible foam and soda straws for our biomedia--yes, the kind the kids like to drink through. We bundle the straws up and weight them down so they don’t float away and then place them in our filter box. Each straw, because of its length and shape, offers a large surface area for the bacteria to congregate (both inside the straw and out). When you multiply the surface area of a single straw by a hundred or more all tied together, you begin to get an idea of just how much beneficial bacteria your biofilter could be generating! Best of all, straws are available everywhere, and they’re dirt cheap!

Of course, plenty of commercially manufactured biomedia products are available, as well, and you may feel more comfortable using some of them.

Once a biofilter converts ammonia into nitrites and nitrites into nitrates, the nitrates (which, you remember, are only mildly toxic to fish) can be removed through regular partial water changes. They can also be removed by large numbers of water plants, which absorb the chemical through their roots while giving off oxygen in return. (We’ll talk more about using plants as vegetative filters in a future piece.)

Since the number of plants required to accomplish this task is far beyond the scope of most pond owners, frequent water changes are still considered the best way to remove excessive nitrates from a pond.